1. Field of the Invention
The present invention relates generally to CRT displays, and more particularly to the expansion of illuminated picture elements therein in order to increase writing speed and eliminate flicker resulting from line pair destruction by higher priority symbols.
2. Description of the Prior Art
Lines written on a CRT display comprise discrete picture elements. Each picture element is either illuminated or not. A line is written, picture element by picture element, from left to right. The entire set of lines written on a CRT display, denoted a frame, comprises two fields. The first field comprises the odd numbered lines which are written from the top to the bottom of the CRT display. The second field comprises the even numbered lines which are also written, after the entire first field has been written, from the top to the bottom of the CRT display. Each of the two fields may be written in one-sixtieth of a second, providing a frame in one-thirtieth of a second. The picture information written on the CRT display is read from an image memory. The image memory comprises a number of addressable storage locations, which for the purposes of this invention, are herein termed "addresses", each of which contains a "1" or a "0" bit. A one-to-one correspondence exists between the addresses in the image memory and the picture elements in the CRT display. A "1" read from an address in the image memory produces illumination of the corresponding picture element in the CRT display. A "0" read from an address in the image memory engenders no illumination of the corresponding picture element in the CRT display.
When a field is written in a relatively slow time, such as one-thirtieth of a second, illuminated picture elements on a given line create a flicker which is disconcerting to a viewer. Flickering, it has been found, can be eliminated by duplicating the illuminated picture elements on an adjacent line. For example, referring to FIG. 1, a line 12 written in one-sixtieth of a second on a CRT display utilizing a binary character generator comprises illuminated picture elements 13, 14, and 15. A viewer perceives the illuminated picture elements 13, 14, and 15 to flicker. The phenomenon is remedied by duplicating the illuminated picture elements 13, 14, and 15 with, respectively, illuminated picture elements 16, 17, and 18 on an adjacent line 19. When the line 12 is written in one field and thereafter the adjacent replica line 19 is written in another field, the viewer does not experience flickering of the picture elements 13, 14, and 15.
Moving symbols in CRT displays possess various priority levels. Such symbols are surrounded by a mask, an invisible rectangle, which erases symbols of lower priority. In this fashion, higher priority symbols are precluded from merging with lower priority ones. However, the obliteration of a portion of a lower priority symbol by a higher priority one can produce flickering, by destroying duplicated illuminated picture elements. Referring to FIG. 2, a lower priority symbol 20 comprises illuminated picture elements 21 and 22 on a line 23 in one field, and illuminated picture elements 24 and 25 on an adjacent line 26 in the other field. As indicated above, the illuminated picture elements 24 and 25 duplicate, respectively, the illuminated picture elements 21 and 22 to prevent flickering. A higher priority symbol 28 comprises an illuminated picture element 29 on a line 30 in one field, and a duplicate illuminated picture element 31 on an adjacent line 32, in the other field. A mask 34 extends three lines above, and surrounds the higher priority symbol 28. Referring to FIG. 3, the higher priority symbol 28 may move upward such that the mask 34 erases the duplicate illuminated picture elements 24 and 25. With the elimination of the duplicate illuminated picture elements 24 and 25, the remaining illuminated picture elements 21 and 22 flicker.
Thus, there is a need for an apparatus, utilized with a CRT display wherein lines are written relatively slowly, for preventing disconcerting flickering on the display resulting from the erasure of duplicate illuminated picture elements by higher priority symbols.